1. Field of the Invention
The present invention relates to molds of golf balls. More particularly, the present invention relates to a mold on the cavity face of which is provided with a large number of pimples for forming dimples.
2. Description of the Related Art
Golf balls have a large number of dimples on the surface thereof. The dimples disrupt the airflow around the golf ball during its flight to cause turbulent flow separation. By causing the turbulent flow separation, separating points of the air from the golf ball shift backwards leading to the reduction of drag. The turbulent flow separation prolongs the gap between the separating point on the upper side and the separating point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. Reduction in drag and elevation of lift force are referred to as “dimple effect”. Excellent dimples disrupt the air flow more efficiently.
In general, golf balls are formed using a mold having upper and lower mold halves each having a hemispherical cavity. Assuming that the upper mold half cavity is northern hemisphere of the globe and that the lower mold half cavity is southern hemisphere of the globe, the upper mold half and the lower mold half are mated on an equator face (plane including the equator). A large number of pimples are provided on the inner surface of the mold, and dimples are formed on the surface of the golf ball by means of the pimples. The dimple has a shape inverted from the shape of the pimple.
Since the molding material (for example, synthetic resin) leaks outside from a parting face of the upper mold half and the lower mold half, a flash is generated along the equator portion on the surface of the golf ball. The flash is generated along the parting line. This flash is ground and removed with a whetstone or the like. Removal of the flash generated inside the dimple is difficult. In order to facilitate the removal of the flash, any dimple is not formed on the equator. In other words, no pimple is provided on the parting face of the mold. A great circle path is formed on the seam of the golf ball obtained with this mold. The great circle path agrees with the equator. When this great circle path agrees with a part where the greatest circumferential rate of the backspin is attained (hereinafter, may be also referred to as “fastest part”), sufficient dimple effect can not be achieved. The dimple effect achieved when the great circle path agrees with the fastest part is inferior to the dimple effect achieved when the great circle path does not agree with the fastest part. The difference between these dimple effects may deteriorate aerodynamic symmetry of the golf ball. The great circle path further impairs the appearance of the golf ball.
US2002-94886 (JP2002-159598) discloses a mold provided with a parting face having a horizontal plane and an inclined plane. In this mold, pimples can be arranged on the equator except for the parting face. This mold provides a golf ball having a non-smooth seam. This golf ball does not have a great circle path. Similar mold is disclosed also in US2004/41297 (JP2004-89549).
U.S. Pat. No. 5,947,844 (JP10-99469) discloses a mold having pins placed on the parting face. The pin forms a dimple on the golf ball. By this mold, a golf ball not having a great circle path is obtained.
U.S. Pat. No. 6,123,534 (JP11-137727) discloses a mold provided with a parting face having bulges. The bulge forms a dimple on the golf ball. By this mold, a golf ball not having a great circle path is obtained.
According to the golf ball obtained with the mold provided with a parting face having a horizontal plane and an inclined plane, the dimples are sparsely provided in the vicinity of the seam. There remains room for improvement of the aerodynamic symmetry of this golf ball. Also in the case of the golf balls obtained with the mold having a pin or a bulge, their aerodynamic symmetry and appearance are still unsatisfactory.
An object of the present invention is to provide a golf ball that is excellent in aerodynamic symmetry and appearance.